JPH0470563B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical fiber coil, and more specifically, to a highly accurate phase modulation type optical fiber coil with small fluctuations in scale factor.

BACKGROUND ART Currently, gyros are used in various fields, particularly as angular velocity sensors for navigation and attitude control of moving objects such as aircraft, flying objects, and automobiles. If you use this gyroscope,
Not only angular velocity, but also data such as orientation can be obtained by integrating it.

Among these types of gyroscopes, optical fiber gyroscopes can be used in any environment without shielding problems because the light and the optical fiber through which the light propagates are not easily affected by magnetic fields or electric fields, and they have no moving parts. Furthermore, it is superior to conventional gyroscopes in terms of minimum detectable angular velocity (sensitivity), drift, measurable range (dynamic range), and scale factor stability. Therefore, it has been attracting attention and being developed in recent years.

Examples of such fiber optic tires include, for example, gear range gear. G. , Bukalo J. A. Others, ``Optical Fiber Sensor Technology,'' International Institute of Quantum Electronics (Giallorenzi TG, Bucaro J.
A.et al “Optical Fiber Sensor Technology”,
IEEE J. of Quantum Electronics) QE−18, No.
4, pp626-662 (1982) and Kurashio and I.
P. Gilles “Optical Fiber Gyroscope” Journal of Physics Electronics Science Instruments (Culshaw
and IPGiles “Fiber Optic Gyroscopes”J.
Phys.E: Sci Instrum.) 16pp5−15, (1983),
Tsubokawa and Otsuka, "Optical Fiber Gyroscope," Laser Research, 11 , No. 12, pp. 889-902 (1983), etc., show this in detail.

(a) Principle of the optical fiber coil The principle of the optical fiber coil will now be explained with reference to FIG. 2.

The light from the light emitting element 10 is transmitted to the beam splitter 1
The optical fiber loop is made by winding the single mode optical fiber 18 many times in the form of a coil, that is, the input is input to both ends of the sensor coil 20, and the signal is input clockwise (CW) and counterclockwise (CCW) to the sensor coil 20. Propagate light. At that time, if the sensor coil 20 is rotating at an angular velocity Ω,
A phase difference Δθ occurs between the clockwise light and the counterclockwise light, and the angular velocity Ω is detected by measuring Δθ.

The electric field strength E _cw of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 20,
_Eccw is expressed as follows.

E _cw = E ₁ sin (ωt + Δθ/2) E _ccw = E ₂ sin (ωt + Δθ/2) However, E ₁ , E ₂ : Amplitude of counterclockwise light and clockwise light ω: Angular wave number of light t: Time Δθ: Phase difference due to the sannyac effect The counterclockwise light and the clockwise light with the phase difference Δθ are combined by the beam splitter 12,
The light is incident on the light receiving element 26. The phase difference Δθ can be determined from the detection intensity of the light receiving element 26.
The phase difference Δθ can be expressed as follows.

Δθ＝4πLa／cλΩ ……(1) however, L: Fiber length of sensor coil a: Radius of sensor coil c: speed of light in vacuum λ: wavelength of light Ω: rotational angular velocity This is called the sanyatsuk effect.

There are various methods of detecting the phase difference Δθ.
Various things have been proposed.

Most simply, when the sum of the counterclockwise light and the clockwise light is square-law detected using a light receiving element, the output I becomes I∝{1+cos(Δθ)}...(2).

Since Δθ is included in cos, this has the disadvantage of poor sensitivity when Δθ is close to 0.

Therefore, an optical mechanism has been proposed in which the phase of either the counterclockwise or clockwise light is shifted by 90° and square law detection is performed. In this case, the output I takes the form I∝{1+sin(Δθ)} (3), so the sensitivity is good when Δθ is close to 0.

However, in order to separate one of the lights, three new beam splitters are required to separate the optical paths. Furthermore, the lengths of the separated optical paths must always be made equal.

Improving the sensitivity when Δθ is close to 0 using an optical detection mechanism as described above has the above-mentioned difficulties.

(b) Phase modulation type optical fiber irons Therefore, many optical fiber irons that attempt to detect Δθ using a dynamic mechanism have been proposed. For example, a phase modulation method, a frequency modulation method, etc. are used. Among them, the phase modulation optical fiber iron is the most superior in terms of minimum detectable angular velocity.

The phase modulation type optical fiber iron is a method in which a phase modulation element is provided at one end of an optical fiber sensor coil, and the phase difference Δθ is determined by measuring the magnitude of the modulation signal.

The phase modulation type optical fiber coil will be explained with reference to FIG. The optical fiber 18
is divided into a part wound to form a sensor coil 20 and a phase modulation part 24 of an optical fiber wound around a phase modulation element 22 such as a piezo element driven at an angular frequency ω _n . There is. The light coupled from both ends of the optical fiber propagates clockwise and counterclockwise within the sensor coil 20 of the optical fiber, exits from the opposite end, and is transmitted by the beam splitter 12.
The light is combined and enters the light receiving element 26.

When the phase modulation element is installed at an asymmetric position with respect to the sensor coil, the light emitted from the light emitting element at the same time passes through the sensor coil and the phase modulation element winding circuit in clockwise and counterclockwise directions, but the modulation Since the times are different, when the output is square-law detected by the light receiving element, a modulated signal appears in the output. Since Δθ is included in the amplitude of the modulation signal, Δθ can be determined by knowing the magnitude of the modulation signal.

For example, if the length of the optical fiber sensor coil is L, the curvature of the fiber core is n, and the speed of light is c, then the time τ required for light to pass through the sensor coil is τ=nL/c...(4) .

Here, it is assumed that the phase modulation element 22 is provided near the input end of the counterclockwise light, and the modulation signal of the phase modulation element 22 is a sine wave with an angular frequency ω _n as described above. When the light emitted from the light-emitting element at the same time is split into clockwise light and counterclockwise light and undergoes phase modulation, the phase difference φ of the modulation signal is φ=ω _n τ=nLω _n /c=2πf _n nL/ c...(5) However, ω _n =2πf _n .

Due to the sannyac effect, the clockwise light and the counterclockwise light have a phase difference of ±Δθ/2, but the phase is further modulated by the phase modulation element. If the amplitude of the phase modulation element is b, the electric field strengths E _cw and E _ccw of the clockwise and counterclockwise lights are E _cw = E ₁ sin {ωt + Δθ/2 + bsin (ω _n t + φ)} ...( 6) E _ccw = E ₂ sin {ωt + Δθ/2 + bsin(ω _n t)} ...(7).

The clockwise light and counterclockwise light having the electric field strength as described above are combined by the beam splitter 12 and square-law detected by the light receiving element 26, so the output S(Δθ, t) of the light receiving element is E _cw , E is proportional to the sum of _ccw squared.

S (Δθ, t) = {E _cw , E _ccw } ² ...(8) Calculating this, S (Δθ, t) = E ₁ E ₂ cos {Δθ + 2bin (φ/2) cos (
ω _n t+φ/2)}+D.C.+(2ω or more)...(9) However, DC means a direct current component.

{2ω or more} means a term with a frequency twice the angular frequency of light. Note that this is 0 because it is not applied to the detector.

becomes. Thus, because of the phase difference φ provided by the phase modulation element, Δθ can be obtained in relation to the amplitude of the modulation signal.

Therefore, DC is omitted and S(Δθ, t) is expanded into a series using the Betzel function. First, equation (9) is expressed as follows.

S(Δθ, t)=E ₁ E ₂ {cosΔθcos[2binφ/2cos(
(ω _n t+φ/2)] −sinΔθsin [2bsinφ/2cos(ω _n t+φ/2)]
...(10) On the other hand, from the generating function expansion of Betzell function, e ^x/2(t-1/t) = _∞ 〓 ^n=-∞ J _o (x)th ...(11) t=e If we put ⁱ 〓, it can be expressed as e ^ixsin 〓= _∞ 〓 ^n=-∞ Jn(x) ⁿⁱ 〓 ……(12). From the expansion of the real and imaginary parts of equation (12), we can obtain the series expansion of the cos and sin parts of equation (10). Divide S (Δθ, t) into these parts and write S (Δθ, t) 0 = (S _c cos Δθ + S _s sin Δθ) E ₁ E ₂ ... (13), then the conversion of θ → θ + π/2 After doing so, J _-o (x)=(-) ⁿ J _o (x) ...(14) However, using the property that n is a positive integer, we write ξ=2bsinφ/2 ...(15) , writing S _c and S _s above, S _c = J ₀ (ξ) + 2 _∞ 〓 ⁿ⁼¹ (−) ⁿ J _2o (ξ) cos2nω _n t
...(16) S _s =2 _∞ 〓 ⁿ⁼⁰ (-) ⁿ J _2o+1 (ξ)cos(2n+1)ω _n t
...(17) becomes. Therefore, once again, S(Δθ, t) is expressed as follows.

S (Δθ, t) = 1/2 (E ₁ ² + E ₂ ² ) + (component over 2ωt) + E ₁ E ₂ J ₀ (ξ) cosΔθ E ₁ E ₂ 2 _∞ 〓 ⁿ⁼¹ (-1) ⁿ J _2o (ξ)cos2nω _n t・cosΔθ E ₁ E ₂ 2 _∞ 〓 〓 ⁿ⁼⁰ (−1) ⁿ J _2o+1 (ξ)cos(2n+1)ω _n t・sinΔθ
...(10)a = DC component +2E ₁ E ₂ J ₁ (ξ) cosω _n t・sinΔθ −2E ₁ E ₂ J ₁ (ξ) cos2ω _n t・cosΔθ −2E ₁ E ₂ J ₁ (ξ) cos3ω _n t・sinΔθ +2E ₁ E ₂ J ₄ (ξ) cos4ω _n t・cosΔθ + higher-order component...(10)b This is the sum of the series of the fundamental wave of the modulation signal ω _n and the harmonic signal.

Using a suitable filter, it is possible to extract the fundamental wave ω _n or any higher harmonic signal. No matter which signal is adopted, cosΔθ or sinΔθ
You can know the size of

In that case, the corresponding Betzell function J _o (ξ)
The amplitude b of modulation by the phase modulation element, the modulation angular frequency ω _n , and the sensor coil transit time τ should be set so that the value of ω n becomes large.

The highest sensitivity can be expected from equation 1 in (17).
This is the next term (n=0), that is, the second term on the right side of equation (10)b. This is the fundamental wave component. If this fundamental wave component is P (Δθ, t), then P (Δθ, t) = 2E ₁ E ₂ J ₁ (ξ) cosω _n t・sinΔθ...
(18). In this way, an output proportional to sin Δθ is obtained, and Δθ can be found by finding the amplitude of the fundamental wave component.

Note that since sensitivity improves when J ₁ (ξ) is maximized, it is set to ξ = 1.8. At this time, the DC component J ₀ (ξ) is approximately 0.

The above is the basic configuration of the phase modulation type optical fiber coil.

Problems to be Solved by the Invention As mentioned above, in the conventional phase modulation type optical fiber coil, the output of the light receiving element is 2E ₁ E ₂ J ₁ (ξ)
angular frequency ω _n using cosω _n t・sinΔθ as a reference signal
Synchronous detection using the square wave of angular velocity output
Obtain 2E ₁ E ₂ J ₁ (ξ)sinΔθ. However, the angular velocity output 2E ₁ E ₂ J ₁ (ξ) sin Δθ is calculated by E ₁ E ₂ and J(ξ) in addition to Δθ.
It also depends on the scale factor. Especially E ₁ E ₂
Even if the light emitting output of the light emitting element is stabilized, it will fluctuate due to fluctuations in the coupling efficiency of light to the optical fiber, fluctuations in the polarization of light propagating in the optical fiber, etc. When the scale factor fluctuates, the output fluctuates even for inputs of the same angular velocity, resulting in an error in the detection of the input angular velocity.

SUMMARY OF THE INVENTION Therefore, the present invention aims to eliminate the fluctuation of the scale factor and provide a highly accurate phase modulation type optical fiber iron.

Means for Solving the Problems According to the present invention, as shown in FIG. an optical fiber 18 which is branched and coupled to both ends and outputs the light propagated in both directions through the sensor coil from both ends; a phase modulator 30 that performs phase modulation by _n ; a light receiving element that receives the bidirectional light propagated through the optical fiber;
The phase modulation type optical fiber coil is equipped with a synchronous detection device that performs synchronous detection upon receiving the output of the light receiving element, and measures the rotational angular velocity from the phase difference between the two directions of light generated when the sensor coil 20 rotates. The light emitting element includes a first light emitting element 10 and a second light emitting element 11 that emit light of different wavelengths, and the light receiving element emits propagating light having the emission wavelength of the first light emitting element 10. The synchronous detection device includes a first light receiving element 26 that receives light, and a second light receiving element 27 that receives propagating light having the emission wavelength of the second light emitting element. A first synchronous detector 40 that synchronously detects the output, a second synchronous detector 42 that synchronously detects the output of the second light receiving element 27, and an output signal of the first synchronous detector and the second An analog divider 49 receives the output signal of the synchronous detector and outputs the ratio of the two output signals.

Function If the wavelength of the first light emitting element 10 is λ ₁ and the wavelength of the second light emitting element 11 is λ ₂ , then the first light receiving element 26
The output of the second light receiving element 27 is set to a frequency 2ω _n which is twice the phase modulation frequency _.
The signals obtained by synchronous detection in the synchronous detectors 40 and 42, respectively, are given by the following equation.

P ₁ ηJ ₁ (ξ)sinΔθ ₁ Δθ ₁ =4πLa/cλ ₁ Ω ...(19) P ₂ ηJ ₂ (ξ)cosΔθ ₁ Δθ ₂ =4πLa/cλ ₂ Ω ...(20) P ₁ : First Output P ₂ of the light emitting element: Output η of the second light emitting element: Loss of the optical system Here, the wavelength λ ₂ of the second light emitting element is twice the wavelength of the first light emitting element, that is, λ ₂ = 2λ. If it is set to ₁ , the relationship Δθ ₁ =2Δθ ₂ is obtained.

Therefore, by taking the ratio of the signal component shown in equation (19) and the signal component shown in equation (20), the following relational expression can be obtained.

P ₁ ηJ ₁ (ξ)sinΔθ ₁ /P ₂ ηJ ₂ (ξ)cosΔθ ₂ =P ₁ J ₁ (
ξ) sin2Δθ ₂ /P ₂ J ₂ (ξ)cosΔθ ₂ =P ₁ J ₁ (ξ) /P ₂ J
₂ (ξ)sinΔθ ₂ ...(21) The signal component ratio shown in equation (21) is the loss η of the optical system.
is erased and is not affected by loss fluctuations in the optical system. In this way, the scale factors are only the outputs P ₁ and P ₂ of the two light emitting elements and the phase modulation parameters J ₁ (ξ) and J ₂ (ξ).

All of these scale factors are extremely easy to control compared to loss fluctuations in the optical system. For example, regarding the output of the light-emitting element, as shown in FIG. 4, a part of the light from the light-emitting element 10 is received by the light-receiving element 28, and the driving current of the light-emitting element 10 is fed back so that the output level does not fluctuate. Easy to control. Further, in phase modulation, an optical fiber is usually wound around a piezoelectric vibrator, and the phase of propagating light is modulated by mechanical strain of the piezoelectric vibrator. Therefore, regarding the parameters of phase modulation, as shown in FIG. 5, a monitor electrode 45 is provided in addition to the drive electrode to monitor the above-mentioned distortion so that the degree of phase modulation is constant. Feedback control is also easy.

Embodiments Hereinafter, embodiments of the phase modulation type optical fiber coil according to the present invention will be described with reference to the accompanying drawings.

Figure 6 shows one example of a phase modulation type optical fiber coil in which the present invention is implemented using discrete components.
FIG. 2 is a diagram showing the configuration of an example. For the minimum configuration with the basic requirements of the fiber optic gyroscope,
Izekiel S. and A.D.H.
J.A. "Fiber Optic Rotation Sensor" Splinigaveerlag Berlin (Ezekil S. and Arditty
HJ “Fiber Optic Rotatior Sensors”, Springer
-Verlag Berlin.) 1982 has a detailed explanation.

In the illustrated phase modulation type optical fiber iron, a first light emitting element 10 and a second light emitting element 1
1 is provided and driven by a power source (not shown) to generate light beams having mutually different wavelengths. Note that a He-Ne laser, a semiconductor laser, a superluminescent diode, etc. can be used as the light emitting element. In this case, for example, an AlGaAs-based semiconductor laser is used as the first light-emitting element and an InGaAsP-based semiconductor laser is used as the second light-emitting element, so that the emission wavelength of the second light-emitting element 11 is different from that of the first light-emitting element 10. It can also be set to twice the emission wavelength. The light beams generated by the two light emitting elements 10 and 11 enter mode filter fibers 16 and 16' via beam splitters 12 and 12', such as half mirrors, respectively. The light beams propagated through the mode filter tough fibers 16 and 16' are combined by a beam splitter 14 via polarizers 15 and 15', and are further split into two and coupled to both ends of an optical fiber 18.

The optical fiber 18 consists of a sensor coil 20 wound into a coil shape many times and a portion coupled to a phase modulator 30 disposed near one end of the sensor coil 20 so as to constitute an optical fiber sensor. There is.

The phase modulator 30 is composed of, for example, a piezoelectric vibrating element, is connected to an AC excitation power source (not shown) for phase modulation, and is driven by a rectangular AC wave having an angular frequency ω _n . In this case, the optical fiber 18 is wound around a phase modulator 30, for example.

The light beam propagating clockwise and counterclockwise through the optical fiber 18 is emitted from both ends of the optical fiber 18, combined by the beam splitter 14, and further split into two, which are sent to the mode filter tough fibers 16 and 16'. incident. Mode Filter Tough Aiba 16
The light beams propagated through the beam splitters 12 and 16' are sent to the light receiving element 2 through the beam splitters 12 and 12'.
6 and 27, respectively. The first light receiving element 26 and the second light receiving element 27 are set to receive only propagated light having the emission wavelengths of the first light emitting element 10 and the second light emitting element 11, respectively. Therefore, for example, the first light receiving element 26
A Si-based photodiode can be used as the photodiode, and a Ge-based, InGaAs-based, or InGaAn-based photodiode can be used as the second light receiving element 27.

The electrical outputs of the light receiving elements 26 and 27 are connected to the inputs of synchronous detectors 40 and 42, respectively, via DC blocking filters. Synchronous detector 40
and 42 synchronously detect the outputs from the light receiving elements 26 and 27 using frequency signals ω _n and 2ω _n , and output voltage signals of components of angular frequencies ω _n and 2ω _n , respectively.

The analog divider 49 calculates the ratio of the electrical output of the synchronous detector 40 and the electrical output of the synchronous detector 42, and indicates the phase difference Δθ generated in the sensor coil 20, as shown in equation (21) above. An angular velocity signal is obtained.

The phase modulation type optical fiber coil configured as described above operates as follows.

The light beams from the light emitting elements 10 and 11 driven by the power source are each passed through a beam splitter 12.
and mode filter tough fiber 16 via 12'.
and 16', are combined by the beam splitter 14, and further branched into two optical fibers 18.
is connected to both ends of the

The light beam input to the optical fiber 18 has a phase difference at the part of the sensor coil 20 undergoing rotation, and also has a phase difference at the part coupled to the phase modulator 30 driven by a rectangular alternating current with an angular frequency ω _n . is phase modulated at

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase-modulated in the optical fiber 18, are output from both ends of the optical fiber 18, are combined by the beam splitter 4, and are further divided into two.
and propagates through mode filter tough fibers 16 and 16', and beam splitters 12 and 1.
The light enters the light-receiving elements 26 and 27 via 2'.

The outputs of the light receiving elements 26 and 27 are sent to the synchronous detector 4.
Synchronous detection is performed at angular frequencies ω _n and 2ω _n at 0 and 42, respectively, and the above equations (19) and (20)
Voltage signals expressed by the following equations are output.
Analog divider 4 calculates the ratio of the above two voltage signal outputs.
sin Δθ calculated in step 9 and expressed by equation (21) above.
An output proportional to , that is, an angular velocity signal is obtained. At this time, as described above, errors caused by loss fluctuations in the optical system are eliminated from the angular velocity signal.

Therefore, in the phase modulation type optical fiber iron, it is possible to significantly reduce fluctuations in the scale factor due to fluctuations in loss in the optical system, thereby ensuring high accuracy.

FIG. 7 is a schematic diagram of the configuration of a second embodiment in which the phase modulation type optical fiber coil shown in FIG. 1 is made all-fiber. The illustrated optical fiber gyroscope is
Fiber couplers 45 to 4 as branching and merging elements
8 and a fiber type polarizer 17 as a polarizer.
The only difference in construction from the optical fiber gyroscope shown in FIG. 1 is that it uses a .

In addition, it is also possible to configure the optical system on a single planar waveguide to create a so-called integrated optical structure.

Effects of the Invention As is clear from the above description, the phase modulation type optical fiber iron according to the present invention significantly reduces the fluctuation of the scale factor, which has been a problem in the past, and ensures high accuracy. Therefore, the phase modulation optical fiber coil according to the present invention can be used in a wide range of applications.

[Brief explanation of drawings]

FIG. 1 is an optical system diagram illustrating the principle of the phase modulation type optical fiber coil according to the present invention.
FIG. 3 is a basic configuration diagram illustrating the principle of an optical fiber gyro, and FIG. 3 is a basic configuration diagram illustrating the principle of a phase modulation type optical fiber gyro.
FIG. 4 is a diagram showing a method of monitoring the light emission output of a light emitting element, FIG. 5 is a diagram showing a method of monitoring the degree of phase modulation, and FIG. 6 is a diagram showing a method of monitoring the degree of phase modulation. FIG. 7 is a schematic diagram of the configuration of a first embodiment of a phase modulation type optical fiber iron according to the present invention, and FIG. (main reference numbers), 10, 11... light emitting element,
12, 14... Beam splitter, 15... Polarizer, 17... Fiber type polarizer, 18... Optical fiber, 20... Sensor coil, 26, 27, 28
... Light receiving element, 30 ... Phase modulator, 40, 42
... Synchronous detector, 45 ... Monitor electrode.

Claims (1)

[Scope of Claims] 1. A device comprising a light emitting element and a sensor coil portion wound in a coil shape many times, and in which light from the light emitting element is branched and coupled to both ends, and light propagated in both directions through the sensor coil is split. an optical fiber that outputs from both ends; a phase modulator that is provided near one end of the optical fiber and that modulates the phase of light propagating through the optical fiber;
It includes a light receiving element that receives the light from both directions propagated through the optical fiber, and a synchronous detection device that receives the output of the light receiving element and performs synchronous detection, and detects the phase difference between the light from both directions that occurs when the sensor coil rotates. In a phase modulation type optical fiber iron that measures a rotational angular velocity from a point in time, the light emitting element includes first and second light emitting elements that emit light of different wavelengths, and the light receiving element detects the light emitted from the first light emitting element. a first light-receiving element that receives propagating light having a wavelength, and a second light-receiving element receiving propagating light having an emission wavelength of the second light-emitting element.
The synchronous detection device includes a first synchronous detector that synchronously detects the output of the first light-receiving element, and a second synchronous detector that synchronously detects the output of the second light-receiving element. and an analog divider that receives the output signal of the first synchronous detector and the output signal of the second synchronous detector and outputs the ratio of the two output signals. Modulation method optical fiber gyro. 2 The emission wavelength of the second light emitting element is twice that of the first light emitting element, and the output signal of the first synchronous detector that synchronously detects the output of the first light receiving element at the phase modulation frequency. The output of the second light-receiving element is synchronously detected at a frequency twice the phase modulation same wave number.
2. The phase modulation type optical fiber coil according to claim 1, wherein the ratio of the output signals of the synchronous detector is used as the angular velocity output. 3 The first light-emitting element is an AlGaAs-based semiconductor laser, the second light-emitting element is an InGaAsP-based semiconductor laser, the first light-receiving element is a Si-based photodiode, and the second light-receiving element is an AlGaAs-based semiconductor laser. Ge-based,
Claim 1, characterized in that the photodiode is selected from InGaAs-based and InGaAn-based photodiodes.
3. The phase modulation type optical fiber gyroscope according to item 1 or 2.